Charles H. Spurgeon on Psalms 141:3

Set a watch, O LORD, before my mouth. That mouth had been used in prayer, it would be a pity it should ever be defiled with untruth, or pride, or wrath; yet so it will become unless carefully watched, for these intruders are ever lurking about the door. David feels that with all his own watchfulness he may be surprised into sin, and so he begs the Lord himself to keep him. When Jehovah sets the watch the city is well guarded: when the Lord becomes the guard of our mouth the whole man is well garrisoned. Keep the door of my lips. God has made our lips the door of the mouth, but we cannot keep that door of ourselves, therefore do we entreat the Lord to take the rule of it. O that the Lord would both open and shut our lips, for we can do neither the one nor the other aright if left to ourselves. In times of persecution by ungodly men we are peculiarly liable to speak hastily, or evasively, and therefore we should be specially anxious to be preserved in that direction from every form of sin. How condescending is the Lord! We are ennobled by being door keepers for him, and yet he deigns to be a door keeper for us. Incline not my heart to any evil thing. It is equivalent to the petition, "Lead us not into temptation." O that nothing may arise in providence which would excite our desires in a wrong direction. The Psalmist is here careful of his heart. He who holds the heart is lord of the man: but if the tongue and the heart are under God's care all is safe. Let us pray that he may never leave us to our own inclinations, or we shall soon decline from the right. To practise wicked works with men that work iniquity. The way the heart inclines the life soon tends: evil things desired bring forth wicked things practised. Unless the fountain of life is kept pure the streams of life will soon be polluted. Alas, there is great power in company: even good men are apt to be swayed by association; hence the fear that we may practise wicked works when we are with wicked workers. We must endeavour not to be with them lest we sin with them. It is bad when the heart goes the wrong way alone, worse when the life runs in the evil road alone; but it is apt to increase unto a high degree of ungodliness when the backslider runs the downward path with a whole horde of sinners around him. Our practice will be our perdition if it be evil: it is an aggravation of sin rather than an excuse for it to say that it is our custom and our habit. It is God's practice to punish all who make a practice of iniquity. Good men are horrified at the thought of sinning as others do; the fear of it drives them to their knees. Iniquity, which, being interpreted, is a want of equity, is a thing to be shunned as we would avoid an infectious disease. And let me not eat of their dainties. If we work with them we shall soon eat with them. They will bring out their sweet morsels, and delicate dishes, in the hope of binding us to their service by the means of our palates. The trap is baited with delicious meats that we may be captured and become meat for their malice. If we would not sin with men we had better not sit with them, and if we would not share their wickedness we must not share their wantonness.